Why does my diastolic pressure raise when I stand up?

You need. Higher BP when standing to supply the brain with blood and overcome effects of gravity....Systolic should also rise.
Variable. Blood pressure in the upright position is regulated by receptors in the neck. They sense a fall in pressure and cause nerve signals to blood vessels throughout the body causing them to contract. This causes resistance to flow and raises the pressure. The pressure rises or falls according to blood volume and hormonal (adrenaline) levels. The latter also cause vessels to contract.